A size comparison of Earth and 55 Cancri e. It is the smallest planet to be detected by a ground-based telescope.
Illustration: R Hurt (SSC)/NASA/JPL-Caltech

Astronomers have detected the silhouette of a planet just twice the diameter of Earth using ground-based telescopes. This is the smallest planet detected with this technique and raises hopes that it could be used to search for Earth-sized worlds capable of supporting life.

The “super-Earth”, known as 55 Cancri e, orbits a relatively nearby sun-like star 41 light years away. As the planet passes in front of the star, it blocks a tiny fraction of the light.

Until now, such a minuscule reduction has only been measurable from space, where the lack of atmosphere allows spacecraft to take more precise readings.

“We are really pushing these techniques to the limit from the ground,” says de Mooij, who used the 2.5-metre Nordic Optical Telescope on the island of La Palma, Spain, to make the detection.

55 Cancri e was discovered in 2004. It is the innermost of five planets known to circle the star and has eight times the mass of Earth. It takes just 18 hours to circle its parent star and is thought to have an atmosphere. The astronomers are now attempting to detect water there.

When the planet is in front of the star, light must pass through its fringe of atmosphere where water vapour and other molecules will absorb some light, causing the starlight to dim more at certain wavelengths than others.

Liquid water is a key requirement for life and would be a marker of a planet’s habitability. 55 Cancri e is unlikely to be habitable: it is so close to its parent star that its temperature is 1,700°C, meaning that any water will be in the form of steam. But this would be a proof-of-concept that could be widely applied in the coming years.

Astronomers expect to discover many super-Earth and Earth-sized worlds in the next five years using two new space telescopes. Tess (Transiting Exoplanet Survey Satellite) is a Nasa mission and Cheops (CHaracterising ExOPlanet Satellite) is a European Space Agency mission. Both launch in 2017.

Once found, these planets will be analysed for chemical composition and habitability. Making the technique viable from the ground would significantly speed up the search for habitable planets and lower costs.

“The concept that you can follow-up the Tess discoveries from the ground is pretty exciting,” said Hugh Jones, an astronomer at the University of Hertfordshire.

Until now, it was thought expensive follow-on space telescopes would be needed. “There are many more telescopes on the ground with a wide variety of instruments than in space, so this is a huge advantage,” says Ray Jayawardhana, professor of astronomy at York University, Toronto.

The Nordic Optical Telescope used in the present study is a modest size by today’s standards and raises the possibility of using larger instruments to analyse smaller planets. “With a large telescope we should be able to push down to detect Earth-sized worlds,” says de Mooij.